The Bayer process is almost universally used for the production of alumina from bauxite ore. The process involves pulverizing a bauxite ore, slurring it in caustic soda solution and digesting it at elevated temperatures and pressures. The caustic soda solution dissolves oxides of aluminum to form an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore (referred to as “red mud”) are then separated from the aqueous phase containing the dissolved sodium aluminate. This separation typically occurs through sedimentation, which is often aided by a flocculant, and filtration. Once separated, alumina trihydrate is precipitated from the aqueous sodium hydroxide and collected as product.
In more detail, the pulverized bauxite ore is fed to a slurry mixer where a caustic slurry is prepared. The slurry makeup caustic soda solution is typically spent liquor (described below) and additional caustic soda. The bauxite ore slurry is diluted and passed through a digester or a series of digesters where, under high pressure and temperature, about 98% of the total available alumina is released from the ore as caustic-soluble sodium aluminate. After digestion, the slurry passes through several flash tanks wherein the pressure of the digested slurry is reduced from several atmospheres to one atmosphere and the temperature of the slurry is reduced from about 200° C. to about 105° C.
The aluminate slurry leaving the flashing operation contains about 1 to 20 weight percent solids, which solid consists of the insoluble residue that remains after, or is precipitated during, digestion. The coarser solids may be removed from the aluminate liquor with “sand trap” cyclones. The finer solids are generally separated from the liquor first by gravity settling aided by a flocculant and then filtration, if necessary. In some cases, the slurry of aluminate liquor leaving the flash tanks is diluted by a stream of recycled washer overflow liquor. Any Bayer process slurry taken from the digesters through a subsequent dilution of the slurry, including the flash tanks, but before the primary settler, is referred to hereinafter as the primary settler feed.
Normally, the primary settler feed is thereafter fed to the primary settler (or decanter) where it is treated with a flocculant. As the mud settles, the clarified sodium aluminate solution (referred to as “green” or “pregnant” liquor) overflows to a weir at the top of the vessel and is collected. This overflow from the primary settling tank is then passed to subsequent process steps.
The clarity of the primary settler overflow is crucial to efficient processing of alumina trihydrate. If the aluminate liquor overflowing the settler contains an unacceptable concentration of suspended solids (at times from about 10 to about 500 mg suspended solids per liter), it must be further clarified by filtration to give a filtrate with no more than 10 mg suspended solids per liter of liquor. The treatment of the liquor collected after the primary settlement to remove any residual suspended solids before alumina trihydrate is recovered is referred to as a secondary clarification stage.
The clarified sodium aluminate liquor is cooled and seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, Al(OH)3. The alumina trihydrate particles or crystals are then classified by particle size and separated from the concentrated caustic liquor. A flocculant is used to aid in this classification and separation process. The very fine particles of alumina trihydrate are returned as the seed crystals and the coarser particles are collected as product. The remaining liquid phase, referred to as “spent liquor,” is then returned to the initial bauxite slurry make up and digestion step and employed as a digestant after reconstitution with caustic.
The settled solids of the primary settler are withdrawn from the bottom of the settler or decanter (and referred to as “underflow’) and then passed through a countercurrent washing circuit for the recovery of sodium aluminate and soda. Overflow liquor from the first washing vessel (or “thickener”) is recycled either as primary settler feed, diluting the slurry as it leaves the flash tanks, and/or it may be passed to filtration along with the overflow from the primary settler.
The partial separation of the red mud solids from the pregnant liquor in the primary settler (or decanter) is expedited by the use of a flocculant. This initial clarification of the pregnant liquor is referred to as the primary settler stage. Flocculating agents, such as liquid emulsion polymers, dry polymers and polysaccharides including starch, are commonly used to improve the separation of the insoluble red mud solids by increasing the rate at which these solids settle, by reducing the amount of residual solids suspended in the liquor, and by decreasing the amount of liquor in the settled solids phase, or underflow. Flocculation performance is critically important in the primary settling stages. Red mud solids comprised mostly of iron oxides (typically at least about 50 weight percent of the red mud solids), together with silicon oxides, calcium titanates, calcium phosphate, aluminum hydroxide, sodium alumino-silicates and other materials, commonly represent from about 5 to about 50 weight percent of the materials of the bauxite ore. Generally, these red muds are composed of very fine particles, which hinder the desired rapid and clean separation of the red mud particles from the solubilized alumina liquor. Improving the rate of separation improves the overall process efficiency and increases the output of alumina production. Improving the clarification of the process liquors reduces the need for filtration and further purification and can also increase alumina production. If the separation of the red mud particles is not clean, the resultant solubilized aluminate liquor will require a more extensive treatment to remove residual solids, and/or the alumina trihydrate recovered will contain levels of impurities that are undesirably high for many end-uses of the alumina.
Relatively low molecular weight polymers containing pendant O-acetylsalicylic acid groups for use in biomedical devices are disclosed in U.S. Pat. No. 5,693,320. An acrylamide/4-acrylamidosalicylic acid solution polymer is disclosed in Intern. J. Polymeric Mater., 1992, 18, 165-177.